Engine exhaust valves control the intake of an air-fuel mixture and the discharge of spent gas in the combustion chamber. Each engine valve includes a valve head and a valve stem extending therefrom. The valve head is located within the combustion chamber. Cycles of tight engagement and separation are repeated between a valve seat insert snuggly fitted in a cylinder head and a contact face on the valve head. The engine valve is required to be resistant to heat, corrosion, and wear because it is exposed to elevated temperatures of 700.degree. C. to 800.degree. C. in a combustion chamber and subjected to repetitive collision against the valve seat insert. Furthermore, today's engines are pushed to operate at higher temperatures, higher peak cylinder pressures, more corrosive environments and highly variable fuel types and quality.
Observation of the failed valves has shown that failure appears to result from two separate and distinct modes: (i) corrosive attack that leads to guttering and (ii) radial cracking along the valve face. Guttering tends to predominate in engines burning diesel fuel; whereas radial cracking is generally observed in gas-burning engines, where engine temperatures are typically higher. Guttering in diesel engines is primarily an oxidation phenomenon along the contact face that is accelerated by the presence of deposits. These deposits, which strongly adhere to the contact face, are formed by the combustion of additives in a lubrication enhancing oil. The oxidized region is inherently brittle and erodes away during repeated cyclic loading during operation. The erosion process accelerates in an avalanche effect until engine performance degrades to the point of failure.
The root cause of radial cracking of valves in gas-burning engines appears to be related in part to the residual stresses associated with a hardfacing weldment. The higher the operating temperatures vis-a-vis diesel engines suggest that high-temperature fatigue initiating at the hardfacing/head interface could also play a role.
The wear resistance problems of engine exhaust valves described above may be addressed by hardfacing or manufacturing engine exhaust valves with a base material that is comprised of the components of the present invention. However, such alternatives would be very expensive.
The present invention is directed to overcoming one or more of the problems set forth above.